Intrinsic Instability of the Hybrid Halide Perovskite Semiconductor CH 3 NH 3 PbI 3 *

Abstract

The organic-inorganic hybrid perovskite CH 3 NH 3 PbI 3 has attracted significant interest for its high performance in converting solar light into electrical power with an efficiency exceeding 20%. Unfortunately, chemical stability is one major challenge in the development of CH 3 NH 3 PbI 3 solar cells. It was commonly assumed that moisture or oxygen in the environment causes the poor stability of hybrid halide perovskites, however, here we show from the first-principles calculations that the room-temperature tetragonal phase of CH 3 NH 3 PbI 3 is thermodynamically unstable with respect to the phase separation into CH 3 NH 3 I + PbI 2 , i.e., the disproportionation is exothermic, independent of the humidity or oxygen in the atmosphere. When the structure is distorted to the low-temperature orthorhombic phase, the energetic cost of separation increases, but remains small. Contributions from vibrational and configurational entropy at room temperature have been considered, but the instability of CH 3 NH 3 PbI 3 is unchanged. When I is replaced by Br or Cl, Pb by Sn, or the organic cation CH 3 NH 3 by inorganic Cs, the perovskites become more stable and do not phase-separate spontaneously. Our study highlights that the poor chemical stability is intrinsic to CH 3 NH 3 PbI 3 and suggests that element-substitution may solve the chemical stability problem in hybrid halide perovskite solar cells .